Gear differentials generally include compound planetary gear sets interconnecting a pair of drive axles to permit the latter to rotate in opposite directions with respect to a differential housing. The drive axles rotate about a common axis; and a pair of respective sun gears (sometimes called "side" gears) are fixed for rotation with the inner ends of the two drive axles, such gears acting as the sun gear members of the compound planetary gear sets. The sun gears are interconnected by so-called "element" or "combination" gears, which form the planet gear members of the sets. The planet gears are usually arranged as sets of meshing pairs, being spaced circumferentially and equidistant about the common axis of the sun gears (e.g., four pairs arranged at 90.degree. intervals or three pairs at 120.degree. intervals); and the planet gears may be mounted for rotation about axes that are variously offset and inclined with respect to a common axis of the sun gears and drive shafts. My invention relates primarily to, but is not limited to, "parallel-axis" differentials in which the planet gears are mounted on axes parallel to the common axis of the sun gears.
The entire planetary gearing arrangement within the differential housing supports opposite relative rotation between the drive axle ends (i.e., differentiation), which is necessary to permit the axle ends to be driven at different speeds. Torque transmitted to the drive axles through the inclined tooth surfaces of the sun/side gears generates thrust forces against gear-mounting bearing surfaces within the differential. (Such bearing surfaces may comprise journals formed in the housing, or may be the ends of bores into which the gears are received, or may be special washers positioned between the end faces or shaft ends of the gears and the housing.) The thrust forces, together with other loads conveyed by the gear meshes in the planetary gearing, produce a frictional resistance to relative rotation between the drive axles, this frictional resistance being proportional to the torque applied to the differential housing. The proportional frictional resistance supports different amounts of torque between the two drive axles to prevent their relative rotation until the characteristic "bias" ratio of the planetary gearing arrangement is reached. Once the frictional resistance is overcome and differentiation begins, the torque difference between the axles is proportioned in accordance with the bias ratio. Differentials that divide torque in a substantially constant ratio between relatively rotating drive axles, are referred to as "torque-proportioning" differentials.
The ability to support different amounts of torque between the drive axles is of great benefit to improving traction capabilities of vehicles. Ordinarily, when one wheel of a vehicle with a conventional differential loses traction, the amount of torque that can be delivered to the other drive wheel is similarly reduced. However, when one wheel loses traction so that there is differentiation between the two axles, torque-proportioning differentials deliver an increased amount of torque to the drive wheel having better traction, such increased torque being determined by the characteristic bias ratio of the differential.
In typical parallel-axis torque-proportioning differentials (e.g., U.S. Pat. No. 2,269,734 to L. S. Powell and U.S. Pat. No. 3,706,239 to A. F. Myers), each planet gear is in mesh with a paired planet gear, and each planet gear in the pair meshes, respectively, with one of the sun gears; and one axial end of each individual planet gear is in mesh with its respective side gear, while its other axial end is in mesh with its paired planet gear. This common form of planetary gear is also used in those parallel-axis differentials which, instead of pairs of planetary gears, mount the planetary gears in a continuous circular mesh around the full circumference of each respective side gear (e.g., U.S. Pat. No. 3,292,456 to O. E. Saari and U.S. Pat. No. 3,738,192 to R. J. Belansky). That is, in most parallel-axis torque-proportioning differentials, the planetary gear pairs mesh with each other at only one of their axial ends, and their respective loads are often carried primarily by only one end of their axial mounting supports.
In general, the helical-tooth planetary gears used in parallel-axis type differentials are usually simpler to manufacture than are the relatively complex planetary gears (which combine spur and worm teeth) used in torque-proportioning designs of orthogonal-axis type differentials. (See U.S. Pat. No. 1,373,657 to J. A. Finefrock.) However, when the latter are made with the same number of sun and planet elements as the former, they usually develop greater frictional resistance between their respective gear meshes and support bearings; and this, in turn, provides greater torque bias and/or increased control over the bias ratio. That is, parallel-axis differentials usually provide relatively lower torque bias and less control over bias ratios.
Torque-proportioning differential arrangements are also used to divide engine torque between the front and rear axles of 4-wheel drive vehicles. In such an inter-axle arrangement, often referred to as a "center-box", the output gears (i.e., sun/side gears) are fixed to the respective drive shafts for each axle. In addition to delivering an increased amount of torque to the axle having better traction, such center-box differentials are sometimes designed with differently-sized output/sun gears to split the relative amount of engine torque being delivered to the front and rear axles (e.g., 60% to rear and 40% to front). In such arrangements, special accommodation is made in order to have the planetary gear sets mesh with each other to interconnect the differently-sized sun/output gears. In two known center-box designs, such accommodation is accomplished by arranging the planetary gears into four or more triplet sets, with the center gear of each set being in mesh with one output/sun gear, and the two outer gears of each triplet set being in mesh with the other differently-sized sun/output gear. In one of these latter center-box arrangements (disclosed in German Application DE 40 23 332), the center gear in each of four triplet sets meshes with the larger output/sun gear, while in the other arrangement (shown in U.S. Pat. No. 5,147,252 to Mace et al.), the outer planetary gears in each of five triplet sets meshes with the larger output/sun gear.
Also, one parallel-axis differential of more recent design (U.S. Pat. No. 5,122,101 to G. B. Tseng), provides such differentials with an increase in frictional surfaces and greater control over bias ratio. In this recent design, the paired planetary gears of each circumferentially-spaced set mesh with each other at two separated areas of engagement. That is, each combination gear of the pair is in mesh with a respective one of the side gears, and each shares two separate and distinct meshing areas with its paired combination gear. For each combination gear, the two meshing portions shared with its paired gear "straddle" the portion of the gear which is in mesh with its respective side gear. Preferably, the shared mesh portions are located at the two axial outer ends of the combination gears. This arrangement also improves the load balance on the planetary gear mounting supports.
In regard to one of the features of the invention, a significant portion of automobiles presently being manufactured throughout the world use so-called "C-clips" for assuring that the axle ends cannot be accidentally withdrawn from the differential (see U.S. Pat. No. 4,512,211 to G. A. Stritzel). In this well-known type of assembly, C-shaped (i.e., partial ring) fasteners are fitted within annular grooves formed near the axle ends after the latter have been inserted through respective journals formed in the differential housing and through a respective one of the sun/side gears.
In order to complete this C-clip assembly, it is necessary to provide space for some relative motion between each axle end and the differential housing so that each axle end can be inserted within the differential case for a sufficient distance to expose the locking ring groove formed in the axle end. Once the C-clip locking ring is installed in place, the axle part is then withdrawn to a desired position for normal driving operation. After this has been done for each respective axle part, it is necessary to insert some means for preventing further axial movement of the axles to maintain them and their respectively captured C-clips in the desired position.
One known prior art reference (U.S. Pat. No. 4,365,524 issued to Dissett et al.) discloses a torque-proportioning parallel-axis differential designed to accommodate C-clip assembly. However, the Dissett differential includes only two sets of planet gear pairs and, as indicated above, such relatively fewer gears provide relatively lower torque bias and less control over bias ratios.
This lack of accommodation for C-clip assembly is apparently due to the relatively lower bias ratios available with parallel-axis designs. Namely, C-clip assembly requires that sufficient space be available within the differential housing to permit the insertion and attachment of the C-clips to the axle ends; and this space requirement can only be met with existing higher-bias parallel-axis designs by the removal of at least one set of the differential's planetary gear pairs. Known designs of parallel-axis differentials cannot afford to lose such a gear set. That is, the loss of such planetary gearing (and the thrust forces and frictional resistance produced by the removed gear meshes) would reduce the differential's available bias ratios below the levels specified for its appropriately practical torque-proportioning use.
Of course, known designs could be significantly enlarged to provide the space requirements of C-clip assembly between existing planetary gear sets, but such enlargement would not be acceptable to the automotive industry which places high priority on space and weight reduction.
My invention overcomes these problems and improves parallel-axis differential design (a) by accommodating C-clip assembly without significant increase in differential size and weight, and/or (b) by providing significant weight reduction and improved lubrication; and it accomplishes these improvements without significantly modifying the differential's torque bias specifications.